The present invention relates generally to lasers. More particularly, the invention relates to amplification of the optical output of a quantum cascade laser.
Lasers have been widely used since their invention, and their use has spread to numerous applications over the years. Many different types of lasers exist, so that it is now possible to produce laser light having numerous desired wavelengths and other characteristics. A particularly interesting form of laser is the quantum cascade laser, which is especially useful for producing laser light in the infrared spectrum. The quantum cascade laser is a semiconductor laser which depends on inter-sub-band transitions to produce photons. The laser has an active region in which the transitions and the production of photons occur. During operation of a quantum cascade laser, electrons in the active region of the laser are excited by injection of a current. The excited electrons achieve a lower energy level by transitioning through a succession of energy states, each state having a lower energy than the previous state. All of these states are created in the conduction band by quantum confinement. Thus, the electron passes in steps through a succession of progressively lower energy levels and releases energy with each transition between energy levels. The design of the band structure makes it possible for the electron to emit a photon of the desired wavelength when injected into specific regions of the device, the radiative transition regions. These regions are alternated with injection/relaxation regions where the electrons lose their energy mainly by non-radiative transitions. The whole active core of the device includes usually 20 to 30 active stages, each formed by a radiative transition region and an injection/relaxation region.
Thus, for example, if an electron passes through 25 stages, it is able to emit 25 photons. The ability to induce a single electron to emit multiple photons allows the quantum cascade laser to produce significant power. In fact, quantum cascade lasers are the most powerful mid-infrared semiconductor lasers.
Prior art quantum cascade lasers are typically multi-pass devices. In a multi-pass laser, photons produced by the laser are partially channeled back into the laser, to stimulate emission of still more photons. High power operation usually requires broad area devices, but the use of prior art broad area devices tends to lead to degradation of the quality of the emitted beam. The use of an optical amplifier with a laser helps to make single-pass operation possible and also makes possible higher power output and improved beam quality, but it is difficult or impossible to use prior art techniques to design an optical amplifier that can be used with reasonable efficiency with a quantum cascade laser. Significant obstacles exist which make it difficult or impossible to design a single-pass inter-sub-band optical amplifier using the techniques of the prior art.
As noted above, quantum cascade lasers produce photons through inter-sub-band transitions of electrons. An optical amplifier produces photons by exciting electrons, for example, by injecting current into the amplifier. Light entering the optical amplifier causes stimulated transitions of the excited electrons. In order to provide optical amplification, significant numbers of the stimulated transitions must produce photons.
Inter sub-band transitions are usually mediated by phonons, and the mediation of these transitions by phonons makes nonradiative transitions much more likely to occur than spontaneous radiative transitions under conditions that typically prevail in an optical amplifier. In an inter-sub-band transition optical amplifier, the vast majority of the transitions are nonradiative, producing no photons. An inter-sub-band optical amplifier designed according to prior art techniques would waste so much current due to the production of nonradiative transitions that the amplifier would be highly inefficient at best and completely ineffective at worst. The lack of any optical amplifier which is practical for use with a quantum cascade laser makes it difficult to achieve the advantages which would be possible if such an optical amplifier were available.
There exists, therefore, a need for an optical amplifier which provides acceptable efficiency in the generation of inter-sub-band radiative transitions and which can be used with a quantum cascade laser, as well a laser for use with such an amplifier and techniques for using the laser and the amplifier together.
An assembly according to the present invention includes an optical amplifier having an optical input and an optical output. The optical output has an area significantly greater than that of the optical input and the geometry of the amplifier is such that the amplifier widens from the optical input to the optical output. The optical amplifier is formed of a layered waveguide structure which achieves quantum confinement of electrons and photons within an active region of the amplifier. A distributed feedback laser is suitably coupled to the optical amplifier at the optical input of the amplifier. The optical amplifier and the laser are suitably formed as a single monolithic structure. Injection of current into the amplifier excites electrons within the amplifier and injection of current into the laser excites electrons within the laser, causing the laser to emit photons which are received at the optical input of the amplifier. The photons received from the laser cause stimulated transitions of the excited electrons within the amplifier. Many of these transitions are nonradiative, that is, they do not produce photons. However, the widening of the amplifier makes available many more excited electrons than would be present in the absence of such widening. Therefore, sufficient radiative, that is, photon producing, stimulated transitions occur that the amplifier emits a significant number of photons as a result of stimulated transitions. The geometry of the amplifier makes it an efficient amplifier for a quantum cascade laser.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings.